Apparatuses for locking the sheet of metal to be shaped are currently used in presses for cold-forming operations, such as pressing and drawing metal sheets.
The sheet of metal must in fact be locked in several points of one of its perimetric portions in order to prevent regions arranged perimetrically with respect to the area to be shaped of the sheet from being moved inappropriately between the die parts, jeopardizing the obtainment of the intended shaping.
Currently known apparatuses generally comprise at least one fluid-operated (gas-operated or hydraulic) actuator for the translational motion of a first sheet presser, which is adapted to support, during an initial step of a shaping operation, the sheet of metal to be shaped.
These fluid-operated actuators are generally constituted by gas cylinder actuators.
The first lower sheet presser supports the metal sheet to be shaped so that it lies above a first lower die part, which is rigidly coupled to the base of the press.
Such first sheet presser is adapted to cooperate with a second sheet presser, which is instead rigidly coupled to the second die part associated with the upper slider of the press.
During the actual shaping operation, i.e., during the descent of the upper ram of the press, the two sheet pressers hold a perimetric portion of the metal sheet closed between them.
The gas cylinder actuators which support the lower sheet presser are part of a circuit for regenerating and supplying the working fluid, i.e., the gas, which is provided with suitable means for the delayed return of the pistons, during the opening of the press.
When the piston of the gas cylinder actuator is pushed downward by the action of the press, the gas, which is intrinsically compressible, is in fact evacuated only partially from the pressurized chamber of the actuator; at the beginning of the step for extraction of the shaped sheet from the press, when the upper ram moves upward again and the actuators are discharged, the compressed gas, which is still present within the chamber of each actuator, expands, acting with a substantially impulsive thrust on the perimetric portions of the freshly stamped metal sheet, causing warping, inflections or other similar deformations thereof or even pushing the entire sheet so that it collides against the upper die part, which has started to rise, causing damage.
Such means for the delayed return of the pistons of the gas cylinder actuators are constituted generally by hydraulic plenums or mixed oil and gas plenums, which are adapted to send pressurized oil, under the thrust of the upper ram of the press, into the gas cylinder actuator, in the cylinder compartment which lies opposite the one in which the gas acts, with respect to the piston.
These means, in addition to being expensive, do not eliminate the problem of the overheating of the gas which flows through the intake and discharge valves, said overheating producing an increase in the pressure that the gas applies within the cylinder of the actuator.
Hydraulic plenums also must be provided with cooling devices, since oil, too, by overheating, changes its properties.
In order to eliminate the drawbacks caused by gas overheating, it is necessary to apply cooling devices which are onerous both in terms of additional costs and in terms of space occupation.
Further, the simple application of the hydraulic plenums is not sufficient to eliminate completely the so-called “spring-back” effect, i.e., the elastic reaction of the compressed gas that has remained in the actuator at the end of the compression of said actuator.
It is in fact necessary to coordinate the operation of the plenum or plenums and of the press so as to anticipate the blocking of the injection of oil into the actuator and avoid even the slightest return of oil into the plenum if the thrust of the gas is simultaneous with, or, worse still, anticipates the closure of the one-way valve.